1. Field of the Invention
The invention relates to the biological treatment of domestic or industrial effluent, and drinking water.
As is known, sewage and sometimes drinking water contain materials in suspension, not only mineral but also organic and nitrogen materials. Sewage also contains dissolved or colloidal materials (particles of less than one micron).
In the present invention, a biological treatment processes has been developed according to which a biomass is developed, that is colonies of bacteria which feed on the organic and/or nitrogen materials contained in the fluid to be treated.
2. Description of the Related Art
The biological treatment processes of sewage can be divided into two major categories.
The first category of biological treatment processes involves using activated sludges after pre-treatment. In a so-called "activated sludge" tank where the untreated water arrives, suitable aeration conditions are maintained to supply oxygen to the micro-organisms and to maintain the expansion of the sludges. Appropriate aeration device are used such as pierced tubes in the bottom of the tank or an aeration turbine situated on the surface.
The effluent which comes from this tank passes into a clarifier. At the bottom of the clarifier some of the sludge is drawn off and recycled into the tank to maintain the biological life therein. The excess sludge is eliminated. Y designates the content of biomass in the water, which is maintained generally in the activated sludges tank at a value normally between 2 and 6 g/l., for example equal to 5 g/l.
In general, the thickening of the sludges is effected without intervention of any granular material. In fact, the so-called "activated sludge" process deals in principle with free bacteria or microorganisms.
According to one variation, it has however, already been proposed to add powdered activated charcoal as a purification adjuvant. This is described for example in patent CH-545.254 and in the article "PAC Process" of Mr. John A. Meidle in WATER/ENGINEERING AND MANAGEMENT, June 1982, pp. 33-36. Charcoal is used mainly because of its adsorption capacities allowing it to adsorb non-biodegradable toxics. Taking into account the high price of activated charcoal, it is important to envisage the most efficient regeneration cycles possible, which in practice are complex and energy-consuming. In particular it is necessary to take into account the cost of heating the charged or loaded activated charcoal to 200.degree. C.
According to other variations it was proposed to use calcium carbonate as a sludge adjuvant or even flue dust which is expected to be lost in the excess sludge that is evacuated.
The performances of such a process by activated sludges depends on the ratio of the mass load of incidental pollution (measured in kg/l.). The performance is expressed as a the biological oxygen demand (B.O.D.) for a given period, to the biomass quantity present in the activated sludges tank. Generally one is interested in the parameter B.O.D.5 which corresponds to the biological oxygen demand of a given weight of biomass for 5 days. The ratio, which readjusts the supply of nourishment to the consumption capacity of the biomass, is measured in kg/kg per day or in d.sup.-1. The higher it is the greater the flow of water supply to be treated and the lower the costs. In practice it varies between 0.1 and 1.
The processes by activated sludges also aim sometimes to treat nitrogen pollution. The pertinent parameter is then the weight of treated nitrogen per kilogram of biomass and per day. This parameter is of the same dimension as above.
The activated sludge techniques are limited by the suitability of the activated sludges, loaded or not, to decantation, which is usually characterized by the MOHLMANN index which corresponds to the volume occupied by one gram of sludge. The lower the index, the better the decantation. In practice the indices recorded vary between a value of 1000 (very bad) and a value of 100 (very good). It is understood that the treatment process can be maintained with contents y of biomass as high as the corresponding MOHLMANN index is low.
In order to increase the performances of an installation it is sought a in the prior art to keep the biomass content in a steady state as high as if possible reaching or even exceeding 10 g/l., but the known processes hardly allow more than 5 to 6 g/l. A priori, it is also sought to reduce the MOHLMANN index.
The second category of treatment processes involves using fixed bacterial. For example the bacteria can be fixed on the granular material of a fluidized bed through which the crude effluent is passed from bottom to top. The maintenance of the bed in a fluidized state without material outflow requires very strict constraints with regard to the supply of the effluent, which may turn out to be incompatible with the size of fluctuations in volume of water to be treated daily. In addition, these fluidized-bed processes present starting difficulties. Finally, the regeneration of the granular material of the bed requires a partial recycling between two levels of the bed, which is energy-consuming. Examples of such process are given by U.S. Pat. No. 3,855,120 and by the article on the OXITRON system published in CHEMING-TECH, vol. 51, no. 6, June 1979, pp. 549-559, Verlag Chemie GmBh Weinheim--Germany.